1. The renal handling of purified human immunoglobulin light chain has been studied with an isolated perfused rat kidney preparation.

2. Human immunoglobulin light chain was freely filtered and largely reabsorbed. Fractional reabsorption was characteristic for each of four light chains and varied between 56% and 86%. No renal tubular maximum for human light chain was obtained.

3. Light chains at concentrations up to 10 times those seen in human myeloma were without effect on glomerular filtration rate or sodium and potassium reabsorption in experiments lasting up to 2h.

4. Filtered and reabsorbed light chain returned ultimately to the perfusion medium, indicating a unique property of the tubular handling of this protein. None of the inhibitors tested (ouabain, frusemide, acetazolamide, probenicid) influenced light chain reabsorption.

5. The results are taken to indicate that light chain reaches the site of the transport enzyme, Na+, K+−dependent ATPase, at concentrations which vary with the nature of the light chain. This may provide a mechanism for renal damage in patients with myeloma, after prolonged exposure.

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